For recreation, Decker competes in ultra-endurance contests. In 2000, he almost killed himself when he took part in the Raid Gauloises, a 520-mile race across the Himalayas (air thick with yak-dung dust infected his lungs). Last April, Decker ran the Marathon des Sables – 152 miles across the Sahara.

A year ago November, I saw Decker at a party in Radnor. Earlier in the day, he had finished the Philadelphia Marathon. The day before, he had completed the JFK 50-miler in Maryland, part of which follows the rugged Appalachian Trail through the mountains. (In my 20s, I used to run the same race; afterward, my legs were so beat I was crippled for days.)

Decker, 33, a personal trainer who lives in Rockville, Md., was once normal. In fact, after a football injury sidelined him in high school, he ballooned. He was so out of shape that when he joined the Army, he flunked the fitness test.

Later, after dropping out of college, he drifted to New Orleans. He worked as a bartender and partied like mad. He drank, took drugs, and ate without restraint. A typical “meal”: 30 or 40 chicken wings at Hooters, washed down with a few pitchers of beer. He began looking like the guy who plays the big, fat, obnoxious fiance on TV.

You can see for yourself by checking out the photo in The World’s Fittest You: Four Weeks to Total Fitness (Dutton, $24.95).

The book is just like its author: plain and straight-on. “No gimmicks, gadgets or gizmos,” Decker said.

The secret to fitness?

“You’re going to have to work,” he said.

I asked Decker about the subtitle, Four Weeks to Total Fitness: “How can anyone become totally fit in just four weeks?”

“You’re not going to look like Arnold or Cindy Crawford,” Decker said. “But you’ll have the tools to construct a better body and to become healthier, not just physically but also emotionally and spiritually. It’s not just about aesthetics or standing in front of the mirror in skimpy underwear and admiring yourself. It’s about getting outside, doing stuff and enjoying your life.”

Decker’s book shows you how, with illustrated exercises, workout plans, nutrition advice (both carbs and fat are good; excess calories are the real enemy!), and recipes. But the most valuable lesson is this: The body is enormously adaptable.

This is both good and bad. On the good side, it’s why, after an acclimating spell, we’re able to tolerate the cold of winter. On the bad side:

“Given a constant load day after day, the body will adapt,” Decker said. “The body will plateau and want to stay at that point.” Result: no growth, no improvement.

His advice: Shock your body.

Exercise scientists call it “nonlinear periodization.” What it means is mixing it up, surprising and challenging your body so it doesn’t get stale, you don’t get bored. Says Decker: “It’s like kicking your own butt.”

You do so by applying what Decker calls “the FIT Equation” (for Frequency, Intensity and Time). By playing with these variables, by constantly shaking things up, you throw your body off guard. It responds by growing stronger, tougher, more resilient, and durable.

Example: When weightlifting, instead of doing five sets of each exercise, do two sets of 12 repetitions, with a two-minute break between sets. Next workout, do three sets of 10 reps, with a one-minute break in between. Or one set of five reps, with a barely manageable weight.

Or do an all-barbell workout. Then an all-machine workout. Then a workout mixing machines and dumbbells. Said Decker: “You not only get better results, you also eliminate the boredom factor.”

Runners can do likewise by punctuating training runs with sprints. Decker breaks up his LSD (long slow distance) by periodically running all out from utility pole to utility pole.

Above all, play. Decker leads an early-morning boot-camp workout. For a body-shocking change-up, he sometimes takes his clients to a rock pile. Using rocks ranging from 20 to 50 pounds, they do presses, curls, crunches, lunges, push-ups.

“I’ve got CEOs, guys making a quarter-million a year, holding rocks and rolling around in the mud,” Decker said. “They love it. We have all kinds of fun!…

“I wake up every day just on fire. I can’t sleep. At 5 a.m., my eyes pop open and I’m ready to run outside and hit the playground. It’s an incredible feeling. And if I can do it, a guy who was 50 pounds overweight, you can, too!”